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A question of physics.....

a simple little question, or is it?

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Martin Harris13/10/2016 14:01:05
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Posted by john stones 1 on 13/10/2016 13:56:03:

Chickens originally gave birth same as humans, they saw a market opportunity and evolved the egg laying to meet a need in the market " Ferenghi 49th rule of acquisition"

Eggsactly - sounds about white - or were you yolking? Shell I get my coat?

Edited By Martin Harris on 13/10/2016 14:02:10

Andy4813/10/2016 14:04:01
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Posted by Donald Fry on 13/10/2016 13:00:30:

Someone said above that rotation of the earth replaced this hyperthetical belt, not so, the earth moves the air at the same pace, allowing the plane to move through the air, and fly.

The air moves at the same speed (roughly) as the ground, just as the air above a treadmill moves at the same speed (roughly) as the air flowing either side of the treadmill. Also the plane is moving at the same speed as the Earth's rotation.

Don Fry13/10/2016 14:17:40
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Andy, not so, the tread mill is moving, no mention of hypersonic wind speeds. And in practical terms such a treadmill can be built, even if I have to invent a warp drive to go to where I can find a competent engineer.

BEB, are you suggesting the undercarriage is frictionless, and rubber had no rolling restistance (john, Google Young's modulus, from deep memory)

Andy4813/10/2016 14:29:37
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Eh? Where did hypersonic winds speeds come in????

The point is, the equation only works in a frictionless system, add in friction and the equation becomes far more complex, and the treadmill speed would not reach infinity as soon as the wheel moves relative to the treadmill.

Cliff Bastow13/10/2016 14:45:29
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No it cant fly, There is no forward motion so no lift.

Think of a car on a rolling road.

Edited By Cliff Bastow on 13/10/2016 14:46:39

Gary Manuel13/10/2016 15:12:51
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Posted by Cliff Bastow on 13/10/2016 14:45:29:

No it cant fly, There is no forward motion so no lift.

Think of a car on a rolling road.

Edited By Cliff Bastow on 13/10/2016 14:46:39

Cars drive the wheels, which of course would drive the conveyor in the opposite direction, so the car would not move.

747s on the other hand, are driven by the thrust from their engines and will accelerate the plane up to take off speed. The plane won't care what it's wheels are doing.

Cliff Bastow13/10/2016 15:18:07
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Where the drive is coming from is irrelevant, if the engines are pushing the plane forwards at 100knots and the conveyor is carrying it backwards at 100 knots then no forward movement so no flight.

If you set a walking machine to run at say 5 mph and you walk on it at 5 mph what happens? you stay in the same place. If you walk slower the belt will carry you backwards and if you speed up you will move towards the front of the belt.

ted hughes13/10/2016 15:20:53
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The plane can't take off.

It can't move forward, so there will be no lift.

Gary Manuel13/10/2016 15:35:55
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Posted by Cliff Bastow on 13/10/2016 15:18:07:

Where the drive is coming from is irrelevant, if the engines are pushing the plane forwards at 100knots and the conveyor is carrying it backwards at 100 knots then no forward movement so no flight.

If you set a walking machine to run at say 5 mph and you walk on it at 5 mph what happens? you stay in the same place. If you walk slower the belt will carry you backwards and if you speed up you will move towards the front of the belt.

No....

If the engines are pushing the plane forwards at 100 knots and the conveyor is pushing the BOTTOM OF THE TYRE backwards at 100 knots. The centre of the wheel is still moving forwards at 100 knots.

All it means is that the conveyor will rotate the wheels at the same speed that the plane (and it's wheels which are attached to it) is moving forewards. As the plane is also physically moving forwards, the wheels will spin at the sum of the plane speed plus the conveyor speed. As these are equal and opposite, the wheels will spin at TWICE the speed of the plane (i.e. 200 knots indicated on the speedometer).

Don Fry13/10/2016 15:57:58
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.Sorry Gary, look at the original statement. We have here a 747, a real machine, its engines are trying to move it forward to its takeoff speed, 260 kph, the belt (hyperthetical design) is spinning the wheels backwards, to keep them stationary, probably in the region of 2600 killometer per Second. The tyres melt. The wheels explode. The bearings fail. I did not design that belt,

Bottoms and tops are irreverent.

It does not move, so no air flows over the wings, and no lift.

Edited By Donald Fry on 13/10/2016 15:59:45

Cliff Bastow13/10/2016 15:59:27
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Gary, you might be right there, I will have to give this some thought! I had not considered the bearings in the wheel and the fact that they can rotate independently, so the conveyor will not carry the plane backwards but just spin its wheels.

ron evans13/10/2016 16:13:50
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OK my turn for some real physics.

1/ the 747 cares not a jot what the wheels do once the brakes are off.

2/ the engines go whooooosh.

3/ the wings lift

4/ the plane flies....Yipee

Cliff Bastow13/10/2016 16:13:55
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I take my hat off to you Gary, you are right I think. just tried it at work with a conveyor and a pair of wheels with bearings on an axle. My boss thinks I have gone mad!

If you put it on the conveyor and run the conveyor with no forward force on the axle it stays in the same place or moves with the belt but much slower then the belt, friction losses I guess.

If I push the axle in the opposite direction to the belt it moves forwards easily and the wheels rotational speed increases.

So I have changed my mind and now say it could take off.

Edited By Cliff Bastow on 13/10/2016 16:22:52

Cliff Bastow13/10/2016 16:21:46
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Now what about if you take the wheels off and fit skis?

No only joking I don't want to stir up anymore trouble lol

Peter Beeney13/10/2016 16:37:12
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Cliff, With the greatest respect but perhaps you might care to read the original question again. ‘The conveyor belt is designed to exactly match the speed of the wheels, moving in the opposite direction’ . You have a fixed speed belt, the question is indubitably using an infinitely variable type. I think you’d find that if you tried to push your wheels forward and the belt’s speed increased to match that forward motion exactly it would be very difficult, if not impossible, to move the axle along the belt.

Just my silly opinion, really…

PB

ted hughes13/10/2016 16:41:44
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It is a hypothetical question, real issues like friction etc. don't come into it.

The plane can't move forward, because the belt is moving backwards to match its forward speed all the time.

Imagine no belt, it is just the runway and the earth moving backwards beneath the plane, increasingly fast- the plane will not move relative to the earth.

Edited By ted hughes on 13/10/2016 16:42:26

Erfolg13/10/2016 16:45:22
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Taking the question as written.

I considered the aeroplane at rest. No movement of the wheels, no movement of the conveyor.

I then thought if the aircraft wheel rotates with a angular velocity, or lets says the wheel rotates to provide 1 linear velocity of 1 mph. The conveyor in principal instantaneously responds, with a opposite linear velocity and the aircraft goes nowhere.

I then thought of the aircraft bombing down the runway, the conveyor is not actuated, that is it is stationary, it could be doing any speed, short of take of. Turn on the conveyor, instantaneously the aircraft comes to a halt. All the passengers and the pilot think what the heck, as the effects inertia and momentum become apparent within the aircraft.

Cliff Bastow13/10/2016 16:48:02
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No Peter because the wheels have bearings, so as the belt speeds up they just rotate faster but have no effect on the planes movement, there is no resistance to moving the plane forwards whatever speed the belt is doing.

Cliff Bastow13/10/2016 16:50:40
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You are thinking as I did at first that the conveyor will move the plane backwards, it wont it just spins its wheels and the plane goes nowhere, then add engine thrust, you now have thrust in a forward direction and no force in the backwards direction, plane accelerates and takes off.

Mike Blandford13/10/2016 17:01:24
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The 747 can take off.

If there is no friction (wheels/conveyor belt), then there is no force to oppose the thrust from the engines, so the the 747 MUST accelerate (Newtons first law).

If there is friction, then the top surface of the conveyor belt will drag the air above it along (opposite of wind sheer!). The resulting airflow over the 747 will then provide drag on the 747 to provide the force to oppose the engine thrust keeping the 747 in the same place (Newtons third law). This airflow will then also provide the lift to get the 747 airborne.

Mike.

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